Some doors of vehicles, such as hatchback doors and sliding doors, can be opened and closed by door opening/closing devices besides by a manual opening and closing operation (refer to Patent Literature 1, for example). Some of the door opening/closing devices assist the manual opening and closing operation, thereby reducing an operator's burden.
Such a door opening/closing device includes an electric motor and an output shaft to which power of the electric motor is transmitted via a deceleration mechanism. In the door opening/closing device, the output shaft is rotated by the drive of the electric motor. As a result, the power of the electric motor is transmitted along the axial center of the output shaft, making it possible to move a door provided to a vehicle main body in an opening direction or a closing direction.